1. Field of the Invention
The present invention relates to a working electrode for electrochemical-enzymatic sensor systems and to a method of producing such a working electrode.
2. Description of Related Art
For the medical detection of metabolic processes in body fluids, for instance, glucose reaction, sensor systems are used in which changes in electrochemical potential which are produced by enzymatic reactions serve as characteristic value. The following requirements are made on the working electrodes of such sensor systems:
electrical conductivity &gt;10.sup.-2 .OMEGA..sup.-1 cm.sup.-1 PA1 variability of shape for special electrode geometries PA1 chemically inert character, particularly with regard to the electrochemically produced reaction products PA1 compatibility of the electrode material with the human body PA1 smooth surfaces without roughnesses or grain boundaries for avoiding the danger of a thrombosis upon the implantation. PA1 a sensor electrode of electrocatalytically inactive carbon PA1 a counter electrode PA1 a reference electrode PA1 an enzyme-containing layer present in front of the sensor electrode, and PA1 a membrane of biocompatible, hydrophilic oxygen-pervious material which covers the enzyme layer off from the body fluid and holds the enzyme back.
From EP-OS 0 470 290 an electrochemical-enzymatic sensor for the determining of substances in body fluids, particularly glucose, is known. This sensor has the following characteristics:
In this sensor, the material for the sensor electrode, which is also known as working electrode or measurement electrode, consists of vitreous carbon, pyrographite, sputtered carbon, sputtered graphite or amorphous hydrogenated carbon. Vitreous carbon is preferred, namely in the form of a smooth vitreous-carbon electrode.
Vitreous carbon is generally produced by pyrolysis of polyfurfuryl alcohol. This material, as carbon modification--is sufficiently inert chemically and, on basis of its amorphous structure, it has a smooth surface. It is suitable for implantation. Vitreous carbon also satisfies the requirements with regard to electric conductivity. However, one disadvantage is that vitreous carbon is difficult to contact. Furthermore, vitreous carbon requires costly processing since cracks can easily occur in thin layers upon the high-temperature pyrolysis involved in its production.